The 31/53 Instrumentation system consisted of a
special TM-500 plug-in module called the 153 Interface, a special TM-500
chassis that contained a power supply for the TM 500 plug-in instrumentation
modules and a backplane for communications between the 153 interface and the
instrument modules. Lastly up to two modified (Option 4) TM-500 instrument
plug-ins, and a cable that connected the 153 interface to the I/O connector
on the rear panel of the Tek 21 or Tek 31 calculator.